1. Field of the Invention
The present invention relates to an information recording medium for recording and reproducing information by means of a laser beam.
2. Description of Prior Art
Information recording media using a beam of high energy density such as a laser beam have been developed in recent years, and are now put into practical use. The information recording medium is generally called "optical disc", and can be used as a video disc and an audio disc as well as a disc memory for a large-capacity computer and a large capacity-static image file. Among these information recording media, a compact disc (i.e., CD) is now widely used as an audio disc for reproducing music or the like. The compact disc is used only for reproducing information of pit rows having been formed on a substrate in the preparation stage of the disc. In other words, the compact disc is prepared by molding an appropriate plastic material to form pits in the spiral form and then providing a metal layer as a reflecting layer on the surface thereof. Accordingly, the compact disc is a recording medium for only reproducing information.
Reading of information from the optical disc is carried out by irradiating the optical disc with a laser beam under rotation of the disc. The information is reproduced by detecting variation of the amount of a reflected light, the variation being caused by presence or absence of the pits on the disc. The compact disc used for only reproduction of information is so prepared that the information is read (reproduced) under rotation of the disc at a fixed linear speed of 1.2 to 1.4 m/sec according to the predetermined CD standard, and CD is required to have a recording time of 74 minutes at longest at a track pitch of 1.6 .mu.m using a signal-recording area within a region between an inner diameter of 46 mm and an outer diameter of 117 mm.
The audio CD is widely used at present as described above. Therefore, commercially available CD players for reproducing information from the audio CD are also widely used, so that the audio CD has been reduced in the manufacturing cost in virtue of mass production and improved also in the quality similarly.
Information recording media of DRAW-type (Direct Read After Write-type) or WO (Write Once) which can be written (recorded) with information have been developed and some of them are put into practical use. The information recording medium (i.e., optical disc) of DRAW-type basically comprises a disc-shaped substrate made of a plastic material or a glass material and a recording layer made of a metal such as Bi, Sn, In and Te, a semi-metal or a dye, which is provided on the substrate. Recording of information on the optical disc can be carried out by irradiating the disc with a laser beam. The irradiated area of the recording layer of the optical disc absorbs energy of the laser beam and rise of temperature locally occurs, and as a result, a chemical or physical change (e.g., formation of pits) is caused to alter (or change) optical characteristics of the recording layer in the irradiated area, whereby information is recorded on the recording layer. Reading (i.e., reproduction) of information from the optical disc is also carried out by irradiating the disc with a laser beam, that is, the recorded information is reproduced by detecting a reflected light or a transmitted light corresponding to the change in the optical characteristics of the recording layer.
Irradiation of the optical disc with a laser beam for recording and reproducing information is generally made on the predetermined area of the disc surface. In order to accurately track the predetermined area for the irradiation by guiding the laser beam, the surface of the substrate is generally provided with a pregroove (i.e., tracking guide) of depressed shape. The pregroove generally consists of a single groove in the form of spiral or a set of plural grooves in the form of circle.
Further, there has been proposed an information recording medium having an inner ROM area (prepit area having prepits) which are previously formed on the substrate, and an outer recording area provided with the above-mentioned pregroove on which pits for reproducing information data are to be formed by irradiation with a laser beam (see: Japanese Patent Provisional Publication 2(1990)-42652). In this information recording medium, a dye layer for recording the desired information is provided on the outer recording area, while the dye layer is not provided on the prepit area. In other words, the information recording medium is provided with a dye layer only on the recording area and not provided with the dye layer on the prepit area (i.e., ROM area). Hence, there is a boundary between the area where the dye layer is provided and the area where the dye layer is not provided. That is, the boundary is formed at the inner periphery portion of the dye layer. When a pick-up generating a laser beam passes over the inner periphery portion of the dye layer, tracking error or focusing error occurs owing to great variation of reflectance on the periphery portion. Further, when the dye layer is formed, a thickness of the dye layer at the periphery portion increase, the shape of the periphery does not become a concentric circle and therefore the shape apt to become ununiform.